1. Field of the Invention
This invention relates to an optical pickup apparatus which is useful in an optical information reproducing system such as a compact disc player and a video disc player.
2. Description of the Prior Art
In an optical information reproducing system such as a compact disc player or the like, an optical pickup apparatus is employed to reproduce information from a recording medium, e.g. a so-called compact disc. It has been proposed to use a diffraction device in such an optical pickup apparatus, thereby reducing the number of components of the optical pickup apparatus.
FIG. 6 shows a conventional optical pickup apparatus having a diffraction device. The optical pickup apparatus of FIG. 6 which obtains optical signals from a recording medium 16 comprises a semiconductor laser device 11 for emitting a laser beam, a diffraction device 13, a collimating lens 14, an object lens 15, and an photodetector 17. The photodetector 17 is disposed at the side of the semiconductor laser device 11.
A laser beam emitted from the semiconductor laser device 11 passes the diffraction device 13 and the collimating lens 14, and is then focused on the recording medium 16 by the object lens 15. The light beam reflected from the recording medium 16 passes again through the object lens 15 and the collimating lens 14, and then enters the diffraction device 13 to be diffracted. The first-order diffraction beam enters the photodetector 17 which converts optical signals incident thereon to an information signal, a focus error signal and a tracking error signal.
When the laser light beam emitted from the laser device 11 passes the diffraction device 13, the major part of the laser light beam propagates in the diffraction device as indicated by the arrows A.sub.4, and a part of the laser light beam is reflected at the surface 13a of the diffraction device 13 as indicated by the arrows A.sub.3. Then, a part of the laser light beam is reflected at the other surface 13b of the diffraction device 13 as indicated by the arrows A.sub.2. The reflected light beams A.sub.2 and A.sub.3 also enter the photodetector 17.
In this way, the photodetector 17 receives the reflected light beams A.sub.2 and A.sub.3 in addition to the light beam reflected from the recording medium 16, resulting in that the output signals of the photodetector 17 are offset or biased.
When only the reflected light beam A.sub.2 or A.sub.3 is to be considered, the degree of the offset produced in the output signals of the photodetector 17 may remain within an acceptable range because the amounts of the reflected light beams A.sub.2 and A.sub.3 are small as compared with that of the light beam reflected from the recording medium 16. In a practical optical pickup apparatus, however, both the reflected beams A.sub.2 and A.sub.3 impinge on the photodetector 17, resulting in an offset which is not negligible. Hence, it has been difficult to properly detect information from the information signal or to accurately conduct focus control or tracking control. This problem may be overcome by disposing, in the vicinity of the photodetector 17, a shield means for preventing the light beams A.sub.2 and A.sub.3 from entering the photodetector 17. However, this causes the optical pickup apparatus to be large in size and weight.
FIG. 7 shows another conventional optical pickup apparatus which is used for the three-beam method. The optical pickup apparatus of FIG. 7 which obtains optical signals from a recording medium 16 is provided with another diffraction device 12 in addition to the diffraction device 13. The photodetector 17 has six photodetecting regions 17a-17f, as shown in FIG. 8. The photodetecting regions 17a-17d convert optical signals incident thereon to an information signal and a focus error signal, and the photodetecting regions 17e and 17f cooperate to produce a tracking error signal.
A laser light beam emitted from the semiconductor laser device 11 is diffracted by the diffraction device 12 to be split into three separate beams; the main beam for producing an information signal and focus error signal in the three-beam method; and two sub-beams for producing a tracking error signal. The major portion of the beams propagates through the diffraction device 13, and then enters the object lens 15 via the collimating lens 14. Thereafter, these beams impinge on the recording medium 16, so that the main beam is focused on a pit of the recording medium 16 and the sub-beams are focused respectively on the positions in front of and behind the pit along the track direction.
The main beam and sub-beams focused on the recording medium 16 are reflected therefrom. Then, the reflected main beam and sub-beams pass again the object lens 15 and collimating lens 14, and are diffracted by the diffraction device 13 so that the main beam is focused on the photodetecting regions 17a-17d, and that the sub-beams are focused on the photodetecting regions 17e and 17f, respectively. The photodetecting regions 17e and 17f generate respectively outputs Se and Sf the level of each of which varies in accordance with the intensity of the sub-beam incident thereon. The tracking error signal can be obtained by calculating "Se-Sf".
In the three-beam method, when the tracking of the recording medium 16 is properly conducted, the sub-beam incident on the photodetecting region 17e is equal in intensity to that incident on the photodetecting region 17f so that the outputs Se and Sf are equal to each other, resulting in that the tracking error signal becomes zero. When the tracking of the recording medium 16 is not properly conducted, the sub-beam incident on the photodetecting region 17e is different in intensity from that incident on the photodetecting region 17f so that the outputs Se and Sf are different from each other, resulting in that the tracking error signal is not zero. The tracking error signal which is not zero causes the track servo control to be activated.
As shown in FIG. 9, the far field pattern of the laser light beam emitted from the semiconductor laser device 11 is an elliptical shape beam the center of which corresponds to the optical axis 11c, the minor axis of which is perpendicular to the optical axis 11c and parallel to the junction plane 11b of the laser device 11, and the major axis of which is perpendicular to the minor axis. Hence, the laser light beams incident on the recording medium 16 have an elliptical shape, but their spots formed on the recording medium 16 are changed in shape in accordance with the positional relationship between the laser device 11 and the photodetector 17.
When the photodetector 17 is positioned so that an angle .theta. formed by the junction plane 11b and the line extending between the laser spot 11a of the laser device 11 and the center of the photodetector 17 is 0 deg. or 180 deg., the spot of each laser light beam forms a long ellipse the major axis of which is perpendicular to the direction along the track (pit train), resulting in that the spot stretches to the adjacent tracks. This may cause the tracking error signal to be erroneously generated. When the photodetector 17 is positioned so that the angle .theta. is 90 deg. or 270 deg., in contrast, the spot of each laser light beam forms a long ellipse the major axis of which coincides with the direction along the track, resulting in inferior resolution while reading the length of the pit. In an actual optical pickup apparatus, therefore, the photodetector 17 is disposed at a position so that the angle .theta. has a value other than 0, 90, 180 and 270 deg., thereby obtaining the information signal in an improved quality.
The laser light beam emitted from the semiconductor laser device 11 is partly reflected by the diffraction devices 12 and 13, a member (not shown) for supporting the optical system, and holders (not shown) for mounting each optical elements, to become so-called stray light which does not contribute to the detection of signals such as the information signal. Particularly, the stray light SL (FIG. 8) caused by the reflection at the diffraction devices 12 and 13 enters into the photodetector 17. The photodetector 17 receiving the stray light SL outputs signals based on the stray light.
As shown in FIG. 8, the optical intensity of the stray light SL distributes in an elliptical shape in the same manner as the laser light beam emitted from the laser device 11. When the photodetector 17 is disposed at a position so that the angle .theta. has a value other than 0, 90, 180 and 270 deg., therefore, the amount of the stray light SL received by the photodetecting region 17e is different from that received by the photodetecting region 17f, causing an offset in the tracking error signal. Even when the tracking control of the recording medium 16 is properly conducted, the photodetecting region 17f receives the stray light SL in a greater amount than the photodetecting region 17e, i.e., the output signal Sf of the photodetecting region 17f becomes greater than the output signal Se of the photodetecting region 17e (Se&lt;Sf), so that the tracking error signal (Se-Sf) is not zero. This causes tracking control to be performed incorrectly.
In order to obtain an optical pickup apparatus reduced in size and weight, it is necessary to position the photodetector 17 in close proximity to the semiconductor laser device 11. As the intensity of the stray light SL in an optical pickup device distributes in accordance with the Gaussian distribution, the smaller the distance between the laser device 11 and the photodetector 17, the greater the offset in the tracking error signal. Hence, it has been difficult to reduce the size and/or weight of an optical pickup apparatus.